The present disclosure relates to a semiconductor structure and a method of forming the same. More particularly, the present disclosure relates to an interconnect structure including a metallic cap located atop a conductive metal feature of the interconnect structure.
Generally, semiconductor devices include a plurality of circuits that form an integrated circuit (IC) fabricated on a semiconductor substrate. A complex network of signal paths will normally be routed to connect the circuit elements distributed on the surface of the substrate. Efficient routing of these signals across the device requires formation of multilevel or multilayered schemes, such as, for example, single or dual damascene interconnect structures. The interconnect structure typically includes copper, Cu, or a Cu alloy since Cu-based interconnects provide higher speed signal transmission between large numbers of transistors on a complex semiconductor chip as compared with aluminum, Al,-based interconnects.
Within a typical interconnect structure, metal vias run perpendicular to the semiconductor substrate and metal lines run parallel to the semiconductor substrate. Further enhancement of the signal speed and reduction of signals in adjacent metal lines (known as “crosstalk”) is achieved in today's IC product chips by embedding the metal lines and metal vias (e.g., conductive features) in a dielectric material having a dielectric constant of less than 4.0.
In semiconductor interconnect structures, electromigration (EM) has been identified as one metal failure mechanism. Electromigration is the transport of material caused by the gradual movement of the ions in a conductor due to the momentum transfer between conducting electrons and diffusing metal atoms. The effect is important in applications where high direct current densities are used, such as in microelectronics and related structures. As the structure size decreases, the practical significance of EM increases.
EM is one of the worst reliability concerns for very large scale integrated (VLSI) circuits and manufacturing since the 1960's. The problem not only needs to be overcome during the process development period in order to qualify the process, but it also persists through the lifetime of the chip. Voids are created inside the metal conductors of an interconnect structure due to metal ion movement caused by the high density of current flow.
Although the fast diffusion path in metal interconnects varies depending on the overall integration scheme and materials used for chip fabrication, it has been observed that metal atoms, such as Cu atoms, transported along the metal/post planarized dielectric cap interface play an important role on the EM lifetime projection. The EM initial voids first nucleate at the metal/dielectric cap interface and then grow in the direction to the bottom of the interconnect, which eventually results in a circuit dead opening.
It has been demonstrated that by replacing the Cu/dielectric interface with a Cu/metal interface can enhance electromigration resistance by greater than 100×. Prior art metal caps are typically comprised of a Co-containing alloy such as, for example, CoWP, which is selectively deposited atop of the Cu conductor region of the interconnect structure utilizing plating, chemical vapor deposition or atomic layer deposition. One problem with utilizing such selectively deposited metal caps is that some of the metal cap extends onto the adjoining surface of the interconnect dielectric material and, as such, electrical shorts between adjacent interconnects may arise.
It is also known to provide a metal cap directly on the surface of an interconnect conductive material, such as, for example, Cu, by recessing the interconnect conductive material below a surface of the interconnect dielectric material. Although this prior art process provides a metal cap that is located only on a surface of the recessed conductive material, such a process is problematic since there is high process cost associated therewith.
It is further known to provide a metal cap utilizing a CuMn alloy which is first deposited within an opening of an interconnect dielectric material and then an anneal is performed to migrate the Mn upwards to form a metallic cap that is composed of Mn atop a Cu structure that has a reduced Mn content. This prior art approach can negatively impact the electrical conductivity of the interconnect structure.